A vehicle air bag system functions to protect a vehicle occupant during a collision. The principal components of a vehicle air bag system are an inflatable air bag and an inflator (or gas generator). At the onset of a collision, the inflator rapidly generates an inert, non-toxic gas (e.g., nitrogen) to inflate the air bag. The inflator is designed to generate a sufficient quantity of gas in a short enough time span to inflate the air bag within milliseconds of the onset of a collision.
A known inflator structure is disclosed in U.S. Pat. No. 4,806,180. The inflator structure comprises a housing containing a gas generant and an initiator for igniting the gas generant in response to a collision signal. The initiator comprises a squib with a housing containing an ignition material and a pyrotechnic material. A bridge wire is located within the squib housing, and is in contact with the ignition material. The bridge wire forms part of an electric sensing circuit. At the onset of a collision, a crash sensor completes the electric sensing circuit, causing electrical current to flow through the bridge wire to ignite the ignition material. In turn, the ignition material ignites the pyrotechnic material. The pyrotechnic material generates hot gases and flame which rupture the squib housing and ignite the gas generant. The gas generant rapidly generates large quantities of a non-toxic, inert gas (e.g., nitrogen), which is expelled from the inflator housing and used to inflate an air bag.
Under certain environmental conditions, an air bag inflator may be exposed to ambient temperatures which are high enough to cause ignition of the gas generant. For example, an inflator could be exposed to such high temperature conditions if it is engulfed by a fire while it is (i) in storage, (ii) being shipped to an installation site, or (iii) installed in a vehicle.
In the air bag art, it is desirable to "set off" an inflator before it is exposed to ambient temperatures which are high enough to ignite the gas generant. Accordingly, it is generally suggested that "auto-ignition" of the gas generant in an inflator occur when ambient temperatures reach a range of about 300.degree.-400.degree. F. This temperature range is below the ignition temperature of gas generant currently being used in vehicle air bag inflators. Moreover, this temperature range is high enough to avoid unintended auto-ignition of the gas generant.
One technique for triggering auto-ignition of the gas generant in an aluminum air bag inflator at a temperature of about 350.degree. F. is shown in U.S. Pat. No. 4,561,675 to Adams, et al., entitled "Auto-ignition Device" and issued Dec. 31, 1985. In the Adams, et al. patent, an electric squib and a pyrotechnic material form a primary ignition train. A package of an auto-ignition material is disposed inside the inflator housing, in good thermal contact with the inflator housing. When the auto-ignition material reaches a predetermined temperature, the auto-ignition material ignites, burns through the package, and ignites the gas generant or the pyrotechnic material to set off the inflator. In the Adams, et al. patent, four factors necessary for effective auto-ignition of the inflator are (i) direct sensing of ambient temperature by the inflator housing, (ii) rapid conduction of heat through the inflator housing, (iii) good heat transfer between the inflator housing and the package of auto ignitable material, and (iv) location of the package of auto-ignition material proximate to the gas generant or the pyrotechnic material within the housing.
Another technique for triggering auto-ignition of the gas generant in an air bag inflator when ambient temperatures are in a range of about 300.degree.-400.degree. F. is shown in U.S. Pat. No. 4,858,951. A homogeneous mixture of booster material and auto-ignition material is incorporated into an initiator housing and forms part of the primary ignition train of the inflator. When the initiator housing reaches a temperature of about 300.degree.-400.degree. F., the auto-ignition material ignites, and sets off the inflator.